The present invention relates to a seal and particularly to an improved high-pressure film-riding circumferential seal for restricting fluid flow between rotating and stationary members, being operable with high pressure differences thereacross and having long lifetime. While usable with all fluid systems, this invention is particularly useful for sealing applications for dry gas environments.
Circumferential sealing devices have gained wide acceptance in a variety of applications, including aero-derivative gas turbine engines and industrial turbo machinery. The use of these sealing devices in the industrial sector usually requires a seal life on the order of five years (43,800 hours) minimum based on continuous operation. These circumferential seals are contacting devices being pressure relievedxe2x80x94not balancedxe2x80x94and their application is usually limited to fifteen to twenty psig thereacross to yield the required life on a typical industrial turbo compressor. The low loading on the seal, which is necessary for achieving the required life, is accomplished by reducing the bore dam sealing length to a reasonably low value. Spring loading, both radial and axial, is likewise controlled by low values. Forces which inhibit the ability of the seal ring to track dynamic motion of the shaft (due to static run-out or vibration) result predominantly from the exposed clearance between the rotating shaft sleeve and the inner diameter of the metallic seal housing. Inertial forces are also present due to the acceleration of the seal ring attempting to track the rotor, but these are generally not a problem as the seal ring is made of a material such as carbon graphite with its low associated density, thus low weight. In the past, attempts to provide a circumferential film-riding seal to achieve these objectives were made by Taschenberg in U.S. Pat. No. 3,575,424, which resulted in a seal ring whose outer periphery was exposed to low pressure, and a minimum clearance was provided by a fixed housing component. Certain prior art structures have formed secondary seals with floating ring components, but these are associated only with non-rotating housing parts, such as the patent to Pope, U.S. Pat. No. 5,284,347; however, while this structure limits the clearance for a secondary seal, no one has provided a floating load ring primary gas seal structure that minimizes the clearance between a rotating shaft and a circumferentially disposed floating load ring.
It is Applicant""s intention to provide a circumferential fluid sealing device (especially useful for gaseous environments, i.e., formed with compressible fluids) having long life and capable of operating with an extremely high pressure drop thereacross, up to 250 psi or more pressure difference, by providing a segmented circumferential sealing ring engaging a floating housing portion, herein called a load ring, mounted in tandem with the segmented seal ring, which serves to help define the clearance between the rotating shaft and the seal. The bore surface of the segmented seal ring is provided with a sealing region adjacent a sealing dam thereon and a bearing region upstream thereof in the direction of fluid flow. In one embodiment of this invention, the bearing section is also provided with lift pockets to move the seal segments away from the rotating shaft (or sleeve mounted on the rotating shaft) to reduce the frictional loads on the seal ring bore surface. Since the seal ring, in accordance with this invention, is made from a lighter weight material such as carbon graphite or a ceramic composite, the sealing components such as lift pockets may be formed in the harder metallic parts such as a metallic shaft sleeve, so that rubbing of the parts does not destroy the lift pockets, thereby reducing the operating life of the seal. The combination of the segmented seal and a floating load ring, together with the employment of lift pockets in a normal bearing region of a circumferential film-riding fluid seal, serve not only to increase the lifetime of the seal but also to permit the seal to operate with higher differential pressures thereacross, which pressure differences (for gas seals) may well be an order of magnitude above the 15-20 psig normally envisioned for seals for these applications having adequate lifetime. This invention also provides a circumferential fluid seal with a reduced seal gap along the seal even with a higher pressure difference across it. In addition, an embodiment is provided wherein the seal ring and the floating load ring are essentially pressure-balanced.